Electric drive systems for machines typically include a power circuit that selectively activates at least one motor at a desired torque. The motor is typically connected to a wheel or other traction device that operates to propel the machine. An electric drive system includes a prime mover, for example, an internal combustion engine that drives a generator. The generator produces electrical power that is used to drive the motor. When the machine is propelled, mechanical power produced by the engine is converted to electrical power at the generator. This electrical power is often processed and/or conditioned before being supplied to the motor. The motor transforms the electrical power back into mechanical power to drive the wheels and propel the vehicle. Some machines having an electrical drive system that utilizes an external source of power during certain modes of operation. Such a machine for example may be an electric drive mining truck. When such a machine is propelled fully loaded and connected to a trolley system, power is fed to the propel motors and converted to mechanical power to drive the machine.
The machine is retarded in a mode of operation during which the operator desires to decelerate the machine. To retard the machine in this mode, the power from the engine is reduced. Typical machines may also include service brakes and other mechanisms for retarding to decelerate and/or stop the machine. As the machine decelerates, the momentum of the machine is transferred to the motor via rotation of the wheels. The motor acts as a generator to convert the kinetic energy of the machine to electrical energy that is supplied to the drive system. This electrical energy is typically dissipated (wasted) across an electrical grid, stored in chargeable cells such as batteries or capacitors for later use, or partially used to power auxiliary components such as blowers for cooling retarding grids.
Some machines, such as some hybrid machines, are configured to store the electrical energy provided by the motor during a retarding mode of operation in energy storage devices or batteries for later use. The stored energy is used to power auxiliary devices and/or drive motors during idling or propel modes of operation so as to minimize engine involvement and reduce fuel consumption. Although such storage configurations may reduce fuel consumption during retarding modes, the extra weight added to the vehicle may in fact increase fuel consumption during propel modes. Implementing storage configurations also introduces significant cost and technological limitations, among other things.
A favored alternative to storage configurations serves to simply waste the energy in the form of heat via a dynamic braking retarding grid of resistors and insulators. To minimize overheating, a grid cooling system having an electrically driven blower is often used to help dissipate the heat from the retarding grid. The blower motor is powered by the waste energy such that the engine is not required to cool the retarding grid. However, retarding grid configurations introduce several control limitations. Among other things, these configurations prohibit operation of the grid cooling system without providing significant braking force. More specifically, because the grid cooling system is powered only by waste energy that is supplied by the motor during retarding modes, the grid cooling system is unable to operate once the machine exits the retarding mode without absorbing a prohibitively large amount of power from the engine and consuming diesel fuel. These systems are susceptible to temperature overshoot conditions, or conditions in which the temperatures of the resistive elements and insulators of the retarding grid sharply increase once a blower is shut off. Furthermore, in low-power retarding modes, or when the retarding arrangement is operating at less than nominal power, the shared DC bus of the drive system may collapse due to the comparatively large retarding requirement. Additionally, these systems still require the engine to be operated at lower RPMs and may reduce fuel consumption, but the engine is still needed to operate other auxiliary devices (i.e. parasitic loads).
Control systems which redirect the electrical energy generated from motors during retarding or braking modes of operation, or regenerative energy, back into the engine are known to those skilled in the art as a means to reduce fuel consumption and improve efficiency. Some existing control systems include a drive system that feeds power generated by traction motors during dynamic braking back into the main alternator to rotate the engine. However, the retarding grids and the grid cooling mechanisms of such systems are linked to the same bus, and thus, cannot be independently controlled. Furthermore, all of these systems specifically require switching of a transfer switch in order to redirect power to the engine during dynamic braking modes.
Therefore, there is a need for a drive system and method that eliminates fuel consumption during certain propel modes and during dynamic braking modes of operation. Specifically, there is a need for an electric drive system and method that automatically and more efficiently redirects power generated at the fraction motor into the engine during dynamic braking modes. There is also a need for an electric drive system and method which provides control of a grid cooling system that is independent from control of the associated retarding grid.